High-resolution and high-sensitivity absorption spectroscopy in external optical resonators with fast frequency tuning

The effect of the laser frequency tuning rate on a weak optical absorption line profile ~(10−5–10−7) cm−1 under conditions when the molecules were in a high-quality optical resonator was studied. The authors used a diode laser and an analytical cavity with two pairs of mirrors with reflectivity of 99% and 99.98% in the ~1.4 μm region. Water vapor at reduced pressure (0.03–1) Torr served as an absorbing medium. A high spectral resolution was obtained by directing laser radiation into the cavity with a small offset relative to its axis (off-axis ICOS). The frequency tuning rate was varied within (102–103) cm−1 s−1. With the increase of the rate, a shift and asymmetry of the Doppler absorption profile were observed. When the tuning direction was changed and the rate was kept the same, the effect preserved in time and mirrored symmetrically on the frequency scale. The measurements were consistent with calculations that took into account the finite lifetime of photons in the cavity and the real ratio of the effective optical path to the coherence length of the laser radiation. Limitations on the frequency tuning rate were discussed using quantitative absorption spectroscopy methods for measuring molecule concentrations. [Display omitted] •The effect of the diode laser tuning rate on an absorption line profile is shown.•Q-factor of analytical cavity strongly influences the line profile distortion.•Tuning rate can negatively affect line profile even from the rates of 8·102 cm−1·s−1.•Theoretical calculations correlate well with the experimental data.